Microporous films are characterized by having excellent permeability and low specific gravity, etc., and thus, the applications thereof cover a wide range including mainly a variety of separators for batteries and electrolytic capacitors, a variety of separation membranes (filters), absorbing articles such as diapers and sanitary products, moisture permeable waterproof materials for clothing and medical purposes, elements for receiving sheet for thermal transfer recording, elements for ink receiving sheets, and polyolefin based microporous films made of such as polypropylene and polyethylene are mainly used.
In general, manufacturing methods for a microporous polyolefin film are roughly divided into a wet process and a dry process. The wet process includes an extraction technique having the processes of adding and dispersing uniformly a material to be extracted into a polyolefin, which is then formed into a sheet, and after that, extracting the material to be extracted with a solvent so that pores are generated in the sheet, and, if necessary, carrying out a stretching process before and/or after the extraction (see, for example, Japanese Patent Nos. 1-299979 A (claim 1) and 3-258737 A (claim 1, page 3, paragraph 2, lines 8 to 20)). The dry process includes a lamellae stretching technique according to which an unstretched sheet with a special lamellar crystalline structure is produced by using special conditions of the crystallization from melt, such as an extrusion at a low temperature and high drafting at the time of sheet formation through the melt-extrusion, and this sheet is stretched mainly uniaxially so that the lamellae interfaces are cleaved and pores are generated (see, for example, Japanese Patent No. 1046436 (claim 1) and Adachi et al., “Chemical Industry,” Volume 47, 1997, pp. 47-52). The dry process also includes an inorganic particle technique according to which an unstretched sheet, where a large amount of incompatible particles such as inorganic particles has been added to a polyolefin, is stretched, and thereby, interfaces between different types of materials are separated so that pores are generated (see, for example, Japanese Patent No. 1638935 (claims 1 to 7) and Japanese Unexamined Patent Publication S60 (1985)-129240 (claims 1 to 4)). Furthermore, there is a β-crystal technique where a β-crystal with a low crystal density (crystal density: 0.922 g/cm3) is generated at the process producing an unstretched sheet through the melt-extrusion of polypropylene, this sheet is stretched so that the crystal is transformed to an α-crystal with a high crystal density (crystal density: 0.936 g/cm3), and pores are generated due to the difference in the crystal density between the two (see, for example, Japanese Patent Nos. 1953202 (claim 1), 1974511 (claim 1), 2509030 (claims 1 to 8), 3341358 (claims 1 to 3) and 3443934 (claims 1 to 5), Japanese Unexamined Patent Publication Nos. H7 (1995)-118429 (claims 1 to 3) and H9 (1997)-176352 (claim 1), Japanese Patent No. 3523404 (claim 1), International Patent Publication Nos. WO 01/92386 (claims 1 to 13) and WO 02/66233 (claims 1 to 11) and M. Xu et a., “Polymers for Advanced Technolgoies,” Volume 7, 1996, pp. 743-748).
According to the above-described β-crystal technique, to generate a large amount of pores in a film after stretching, it is necessary to selectively generate a large amount of β-crystals in the unstretched sheet before stretching. Therefore, it is important in the β-crystal technique that β-crystals are formed under specific conditions of crystallization from the melt using a β-crystal nucleating agent. In recent years, a material having a higher β-crystal forming ability, in comparison with a quinacridone based chemical compound that has been used for a long time (see, for example, Fujiyama, “Polymer Processing,” Volume 38, 1989, pp. 35-41), has been proposed as the β-crystal nucleating agent (see, for example, Patent Documents 16 to 18), and thus, various microporous polypropylene films have been proposed.
For example, manufacturing methods for a resin composition, a film and a porous film, which comprise 0.01 wt % to 10 wt % of ultra-high molecular weight polyethylene or polytetrafluoroethylene and have a β-crystal content (K value) measured by X-rays of not less than 0.5 and a melt tension (MS) measured at 230° C. of no greater than 5 cN for the purpose of improving the film formability at a low temperature and the thickness uniformity of a microporous polypropylene film obtained by a β-crystal technique not less than have been proposed (see U.S. Pat. No. 6,596,814 (claims 1 to 31, page 2, paragraph 1, lines 18 to 50, Examples 1 to 3, Comparative Example 4)).
Alternatively, it has been already known that the β-crystal technique is excellent in comparison among other techniques for lowering the specific gravity of polypropylene through the generation of voids or pores. A method for generating so-called isolated voids, in which neighboring voids are not connected each other, by adding inorganic particles or organic particles or resins immiscible with polypropylene into a polypropylene so that an unstretched sheet is produced, and stretching this sheet so that the interfaces between different types of materials are separated is known as a technique for lowering the specific gravity of polypropylene in addition to the β-crystal technique (see, for example, Japanese Patent No. 2611392 (claims 1 and 2, page 4, paragraph 1, line 40 to page 5, paragraph 2, line 5)). The specific gravity of cavitated films obtained by these other techniques as described above is approximately 0.6 to 0.8 at the lowest while the film with the specific gravity of approximately 0.3 to 0.4 could be obtained by the β-crystal technique without using particles or immiscible resins, depending on its manufacturing conditions (film forming conditions). A white biaxially stretched polypropylene film with the specific gravity of 0.6 to 0.76 has been proposed, which consists of a polypropylene satisfying a specific relationship between the melt strength (hereinafter, referred to as MS) and the melt flow rate (hereinafter, referred to as MFR) and has a layer consisting of a polyolefin based resin (layer B) laminated on at least one side of a cavitated layer (layer A) for the purpose of generating a large amount of the above-described isolated voids uniformly (see Japanese Unexamined Patent Publication No. 2004-160689 (claims 1 to 16, Examples 1 to 10)). In this patent, examples are disclosed where white biaxially stretched polypropylene films with a specific gravity of 0.6 to 0.76 are manufactured by adding a void initiator such as inorganic particles or organic particles or immiscible resins into the layer A with the above-described embodiments and adding optionally a β-crystal nucleating agent so as to generate voids.
However, microporous films obtained by the conventional β-crystal technique are inferior to those obtained by so-called extraction technique and lamellae stretching technique in the permeating performance (hereinafter sometimes simply referred to as “permeability”) of a variety of media. That is to say, microporous films using an highly active β-crystal nucleating agent as shown in Japanese Patent Nos. 2055797 (claims 1 to 8), 3243835 (claims 1) and 3374419 (claims 1 to 4), and those obtained by a β-crystal technique as proposed in Japanese Patent Nos. 1953202 (claim 1), 1974511 (claim 1), 2509030 (claims 1 to 8), 3341358 (claims 1 to 3) and 3443934 (claims 1 to 5), Japanese Unexamined Patent Publication Nos. H7 (1995)-118429 (claims 1 to 3) and H9 (1997)-176352 (claim 1), Japanese Patent No. 3523404 (claim 1), International Patent Publication Nos. WO 01/92386 (claims 1 to 13) and WO 02/66233 (claims 1 to 11) and M. Xu et a., “Polymers for Advanced Technolgoies,” Volume 7, 1996, pp. 743-748 are inferior to microporous films obtained by a extraction technique and a lamellae stretching technique. Therefore, it has been said that microporous film obtained by a β-crystal technique is difficult to expand its application to a high-value added field represented by applications such as a filter or a battery separator or the like where a high permeating performance is required.
In addition, microporous polypropylene films obtained by the conventional β-crystal technique are equal to or slightly superior to those obtained by an inorganic particle technique in the permeating performance and are not significantly characterized as compared to those obtained by an inorganic particle technique, which has excellent cost competitiveness though having disadvantages such as the contamination of the manufacturing process due to dropping off of the particles.
There is also a problem with the β-crystal technique where the productivity is low due to its specific conditions of crystallization from melting at the casting process, where an unstretched sheet is produced. More concretely, in the β-crystal technique, to form a large amount of β-crystals in an unstretched sheet so as to obtain a highly permeable microporous film, polypropylene comprising a β-crystal nucleating agent is used, and in addition, preferably, this is solidified in an atmosphere at a high temperature exceeding 100° C. to obtain a sheet (see for example, International Patent Publication No. WO 02/66233 (claims 1 to 11)). Moreover, there is a report that the lower the melt-extruding temperature is; the greater the amount of β-crystals can be formed (see Fujiyama, “Polymer Processing,” Volume 38, 1989,pp. 35-41). Therefore, the line speed to produce a microporous film is determined by the state of solidification and crystallization of melted polypropylene at the casting process. That is to say, a problem arises where it is difficult to take the sheet off from the metal drum when high speed casting is carried out for a high speed film formation because the sheet in an unsolidified state is sticky. Even in the case where the sheet can be taken off from the drum, the sheet is sometimes elongated at the subsequent process carrying the sheet under tension, and therefore, the casting speed, and thus, the line speed (that is to say, the film formation speed) inevitably decreases and the productivity deteriorates. In addition, in order to make the film exhibit a permeating performance, it is necessary to produce the film by stretching at a temperature lower than that in the conventional stretching conditions for a general-purpose polypropylene film with no permeability in the subsequent stretching process. A problem arises in this stretching process depending on the conditions where the film breaks sporadically, and thus, the production cost further increases.
Furthermore, since an ultra-high molecular weight component of polyethylene or polytetrafluoroethylene sometimes separates out as a coarse gelled substance at the melt-extruding process in the microporous film obtained by the β-crystal technique disclosed in U.S. Pat. No. 6,596,814 (claims 1 to 31, page 2, paragraph 1, lines 18 to 50, Examples 1 to 3, Comparative Example 4 and significantly deteriorates the film formability, it is extremely difficult to realize a low specific gravity and a excellent permeability due to a high β-crystal content and to improve the film formability and the thickness uniformity at the same time.
Moreover, since a void initiator is substantially added to the white biaxially stretched polypropylene film disclosed in Japanese Patent No. 2611392 (claims 1 and 2, page 4, paragraph 1, line 40 to page 5, paragraph 2, line 5 and the temperature of the metal drum to solidify an unstretched sheet at the casting process is low, it is difficult to further lower the specific gravity.
In addition, a polypropylene film with a further lower specific gravity or higher permeability is required and there is a limit in the range of the specific gravity that can be controlled in accordance with the conventional β-crystal technique. Meanwhile, physical properties of the film represented by the Young's modulus and the strength are substantially impaired even if the specific gravity can further be reduced, and therefore, a problem arises where the film get elongated due to the processing tension during the subsequent converting process.
It would therefore be helpful to provide a microporous polypropylene film with a low specific gravity and an excellent productivity, where the permeating performance can be optionally and extremely improved and the physical properties and the dimensional stability are excellent. It would also be helpful to provide a manufacturing method of a microporous polypropylene film with a low specific gravity and an excellent productivity, where the permeating performance can be optionally and extremely improved and the physical properties and the dimensional stability are excellent.